Various distortion techniques for digital content such as video and audio data are described in the prior art. Such distortion techniques may render the content apparently completely disordered, or may leave it understandable to some extent, depending on what is required by the party distributing the content. Such distortion techniques are often referred to as scrambling, and tend to differ from encryption in that scrambling techniques are more vulnerable to brute force reverse engineering attacks. In most practical applications, the scrambling is added as a further layer of protection or control in addition to encryption.
U.S. Pat. No. 7,050,588 describes a content distortion technique that is applied before a potentially lossy content compression step. After transmission to a receiver, and decoding by the receiver, a receiver restore module corrects the distortions using some control parameters generated during the distortion process. In order to reduce bandwidth the compression scheme may typically remove content elements the effect of which is subsequently undetectable to a human playing back the decompressed content. However, due to the lossy nature of the compression, the restore module operates on slightly different data to that output by the content distortion step, which may lead to residual errors in the content output by the receiver that a human will detect.
Content distortion usually also makes subsequent compression and decompression less efficient. This is because spatial and temporal correlations in the original content which compression techniques take advantage of are reduced or lost. To improve the effectiveness of content distortion techniques, therefore, a tight integration with existing encoder and decoder technologies is often desirable, complicating deployment and the use of hardware acceleration techniques in existing content decoders. Many content distortion techniques are therefore designed to work within or after the compression step, for example by changing the signs or order of DCT coefficient and motion vectors in an MPEG elementary stream.
The invention address problems and limitations of the related prior art.